Aperture mask coating to prevent cathode poisoning



June 21, 1960 J. L. SHELDON 2,942,130

APERTURE MASK COATING TO PREVENT CATHODE POISONING Original Filed Feb. 25, 1957 INVENTOR. JOHN L 51/51. 0 ON am re. 0.17

A TTORIVEY 2,942,130 Patented June zl, 1860 APERTURE MASK COATING To PREVENT CATHODE POISONING John L. Sheldon, Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Original application Feb.; 25,1957, Set- No. 642,07 l,

now Patent N; 2,886,730, dated May 12,1959. D1- vided and this application Dec. 12, 1958, Ser. No. 779,896

invention relates to an improved perforate mask for assembly in a cathode ray tube of the type used in multicolor television receivers and to a cathode ray tube embodying such mask.

Certain types of multicolor television systems operate on the general rinciple or emitting 'a Stream of electrons from one or more electron guns mounted in the neck of a cathode ray tube and directing such stream of electrons through a perfoiate mask onto a viewing screen com= posed essentially of a phosphor pattern deposited on a face plate or panel forming the opposite end of the tube. The perforate mask, for example the present tube component commonly referred to as a shadow or aperture mask, is positioned transversely of the electron path intermediate the electron gun and screen to intercept all electrons striking the mask and thereby form a plurality of electron beams passing through the apertures or perforations in the mask to the pattern of hosphors oh the face of the tube. Multicolor television systems of this sort are described in detail in the patent literature, such as United States Patent #2 ,625,7 34 to Harold F. Law.

Theapertures in sucha mask are small and closely spaced, and must be formed with an extreme degree of accuracy to produce a clear, well-denned picture. Thus specifications of "a typical mask call for. apertures mils in diameter and uniformly spaced 2'8 mils center to center and a mask thickness of 55 mils, with the aperture dimensions being held Within a tolerance of i0.5 mil. The inherent difliculties in producing and handling such a mask are quite readily apparent. Furthermore, it is frequently necessary to sag or otherwise form such a mask to a definite curvature.

Etched metal masks have been proposed, but their use requires a great deal of care to avoid Warpage, bending, or other physical damage during production and subsequent handling. It has been recognized that a mask composed of a ceramic material such as glass should be tubes embodying such masks in their assembly tended to fail shortly after being placed on life test. Extensive investigations into these tube failures indicated that the failure was due to a foreign material poisoning the cathode of the electron gun during its operation and that this foreign material was in some manner coming from the glass mask. It is then a primary purpose of the present invention to provide chemically perforated glass masks free from such prior deficiencies and capable of being used in cathode ray tubes for absorption of electrons.

By way of illustration the present invention is described with reference to the accompanying drawing in which,

Fig. 1 is a schematic representation, largely in section, of a cathode ray tube and,

Fig. 2 is an enlarged fragmentary view in section of a perforate mask in accordance with the invention.

In accordance with the present invention, satisfactory glass aperture masks are produced by providing the elec tron intercepting surface of a chemically perforated glass mask with a metal coating. By electron intercepting surface" I mean the glass surface facing, and therefore exposed to, the electron emission gun and hence to the stream of electrons emitted therefrom.

V The cathode ray tube, schematically represented in Fig. 1 and designated by the numeral 19, has an electron gun 12 mounted in the tube neck '14 and a viewing screen 16 composed of a phosphor coating 18, of suitable composition and pattern for multicolor televising, applied to the inside surface of transparent face plate or panel 20 which forms the opposite, enlarged end of the tube. The phosphor coating is applied in any known manner in a dot, stripe or other type of pattern suitable for the purpose. Mounted intermediate electron gun 12 and screen 16, 'by any suitable conventional means not specifically shown, is perforate glass mask 22, having a metal coating '24 on the side of the mask facing gun 12, that is the electron intercepting surface of the mask.

In Fig. 2 a "fra ment or mask 22 is shown greatly exagerated to better illustrate chemically perforated a ertures 26 which are too mall and too numerous to be satisfactorily shown in a full view such as Fig. 1, and to bet"- ter illustrate, as well, metal coating 24. In accordance with the exemplary mask specifi ations referred to earlier, the diameter of apertures 26 is '10 mils and/they are spaced 28 mils part center it) center.- F01 PUIPOSQ'S Of illustration mask 22 is shown flat and apertures 26, as shown, are formed normal to the electron interc pting surface. As a practical matter mask 22 will frequently have imparted to it a definite curvature and a ertures :26 may be formed at angles up 'to 25* or so from normal depending on the location and geometry of the tube components. The angle of each aperture Will be such as to provide alignment of electron gun, apertures and screen elements for proper screen scanning.

In accordance with my invention the electron intercepting surface of mask 22 is coated with a layer of metal 24 of sufficient thickness that electrons emitted by gun 12 are absorbed by the metal and do not penetrate to the glass to any appreciable extent. The exact effect of electron bombardment on chemically perforated glass has not been positively ascertained. It is my belief, however, that absorption of electrons by this particular typeof glass results in a reaction Within the glass accompanied by evolution of a gas which poisons the electron gun cathode, and there is experimental evidence indicating that the offending gas is oxygen. In any event it has been found that, if the glass is shielded by an absorbing layer of metal, poisoning of the cathode is either eliminated or so minimized that normal tube life can be expected.

Various methods of applying metal to glass surfaces are available and may be used in coating glass masks. The process of thermal evaporation is a convenient one since it is well developed commercially to provide an adherent metal coating of controlled and adequate thickness on a glass surface. When this coating method is employed I prefer aluminum as the coating metal since it is satisfactory for electron interception, and thermal evaporation of it is an established and well-understood process in the manufacture of cathode ray tubes. However, other inert metals such as iron, nickel, and alloys of these metals may also be satisfactorily used and in some instances mayev enibe' preferable because of greater density.

Thermally evaporated aluminum films have been heretofore employed in cathode ray tube production as a conducting layer in conjunction with the phosphor screen on the tube face plate.- Such a conducting film extends 'to anelectrical contact or terminal through the tube wall and serves as a metallic conductor to drain electrons from the screen and return them to the power supply. The thickness of such conducting aluminum films is generally on the order of 3000 to 5000 Angstrom units and is minimized to avoid producing any substantial absorption elfect on the stream of electrons directed at the screen. It has also been proposed, as in United States Patent #2,777,084 to James M. Laiferty, to employ such conducting films as electrodes to control electron beam paths.

The thickness of metal required on the glass mask for electron absorption purposes is of considerably greater magnitude and depends primarily on the voltage drop from the electron gun to the tube screen, that is the .velocity of electrons to be intercepted, and on the effective density of the metal coating employed, which in turn depends on the particular metal used. By way of specific illustrative example, a glass mask, designed for use in a cathode ray tube to operate at a rated voltage of kilovolts, was provided with an aluminum film having a thickness of 75,000 Angstrom units and the resulting tube found to perform satisfactorily during a life test of 1000 hours which is considered to indicate a satisfactory tube lower operating voltage than 25 kilovolts were being used, the minimum thickness might be correspondingly reduced. Likewise with another metal having, for example, an effective density, as deposited, three times that of aluminum, the minimum thickness would be onethird, or about 25,000 Angstrom While the invention has been described essentially with relation to what is currently known as a shadow mask, it will be appreciated that it is not restricted to a mask having the specific size and pattern of perforations in such mask but is equally applicable to other types of perforate masks as well regardless of perforation pattern or size. I I a I This application is a division of applicationSerial No. 642,074 filed February 25, 1957 now Patent 2,886,730,

What is claimed is: r

1. An electron absorbing mask for use in a cathode ray tube operating at a predetermined voltage drop which comprises a glass sheet that evolves a cathode poisoning agent when subjected to electron bombardment, an electron absorbing coating on the electron intercepting surface of the glass sheet that consists of a chemically inert metal and has an electronabsorbing power corresponding to that of an aluminum coating onthe orderof. at least 75,000 Angstrom units in thickness, and a pattern of perforations formed through the metal coatedglass sheet to permit selective transmission of electrons.

2. A mask in accordancewith claim 1 in which the metal coating is composed of aluminum and is at least 75,000 Angstrom units thick. e

3. In producing a perforate mask for intercepting electrons in a cathode ray tube operating at a predetermined voltage drop and wherein said mask embodies a perforate glass sheet that evolves a cathode poisoning agent when so bombarded with electrons, the improvement which comprises applying to the electron intercepting surface of said glass sheet an electron absorbing coati'ngof' a chemically inert metal in a thickness corresponding in electron absorbing power to an aluminum coating on the order of at least 75,000 Angstrom units.

References Cited in the file of this patent UNITED STATES PATENTS Hinsch Dec. 31, 1940 ve an 

1. AN ELECTRON ABSORBING MASK FOR USE IN A CATHODE RAY TUBE OPERATING AT A PREDETERMINED VOLTAGE DROP WHICH COMPRISES A GLASS SHEET THAT EVOLVES A CATHODE POISONING AGENT WHEN SUBJECTED TO ELECTRON BOMBARDMENT, AN ELECTRON ABSORBING COATING ON THE ELECTRON INTERCEPTING SURFACE OF THE GLASS SHEET THAT CONSISTS OF A CHEMICALLY INERT METAL AND HAS AN ELECTRON ABSORBING POWER CORRESPONDING TO THAT OF AN ALUMINUM COATING ON THE ORDER OF AT LEAST 75,000 ANGSTROM UNITS IN THICKNESS, AND A PATTERN OF PERFORATIONS FORMED THROUGH THE METAL COATED GLASS SHEET TO PERMIT SELECTIVE TRANSMISSION OF ELECTRONS. 